Particleboard is typically made by first forming a relatively thick mat of the particles--chips or fibers mixed with a heat-activatable resin binder and possibly even rigid laminae--and then simultaneously pressing this mat workpiece while heating it to plastify the resin and reduce its thickness. During this pressing operation, the mat is subjected to a first compression-heating stage and then to a calibration-curing stage. During the first stage the simultaneous application of heat and pressure plastify the resin in the mat and compress the mat to densify it and mix the binder with the particles or fibers. In the second stage the board is merely confined between the two press surfaces which are at an exactly established spacing from each other so that as the resin cures this dimension is accurately imparted to the workpiece. Thereafter the workpiece is cut to size and, if necessary, finished.
In this type of arrangement the system is set up so that every portion of the mat is continuously subjected to compression in accordance with a so-called press curve that has been established in the industry. Since continuous, belt-type presses are relatively new, as compared to the older discontinuous-production multiplaten systems, the pressing curve that was applied via these older presses has been adapted to the newer belt-type continuous-production arrangements. Thus the pressure/time relationship of the discontinuous presses is converted into a pressure/displacement relationship, as continuous uniform displacement is a direct function of time, and the mat is subjected to the appropriate pressure levels in the appropriate regions of the press. In U.S. Pat. No. 4,468,188 of Klaus Gerhardt this distribution of pressing force is achieved by using identical hydraulic rams all pressurized from the same source, but physically distributed in accordance with the desired pressure distribution.
The mat thickness and density as well as the composition of the fibers and binder vary statistically within a certain range, so that when a fixed pressure level is used in the compression zone some portions of a given workpiece can well be subjected to excessive pressure which will damage fibers while other regions will not be compressed enough so that weak voids will be left in the finished product. As such an improperly compressed workpiece passes into the calibration zone it loses its elasticity and becomes mainly plastic, and the defects are made permanent in the product.
German Patent Document No. 2,343,427 proposes a complex control system for dealing with this problem. The calibration zone is provided with strain gauges that measure the pressure with which the mat being calibrated resists compression. A controller compares these detected reaction pressures with desired values so that, for instance, when the reaction pressure drops to indicate the board is overly compressed, it reduces pressure upstream in the actuators bearing on the workpiece in the compression zone. Such a complicated arrangement operates adequately with slowly varying workpieces, but the feedback nature of its operation creates a response time too long to compensate out localized irregularities, and in fact can damage the workpiece in response to detection of such a localized problem.